Diamond-Like Carbon Electronic Devices and Methods of Manufacture

ABSTRACT

Materials, devices, and methods for enhancing performance of electronic devices such as solar cells, fuels cells, LEDs, thermoelectric conversion devices, and other electronic devices are disclosed and described. A diamond-like carbon electronic device can include a conductive diamond-like carbon cathode having specified carbon, hydrogen and sp 2  bonded carbon contents. In some cases, the sp 2  bonded carbon content may be sufficient to provide the conductive diamond-like carbon material with a visible light transmissivity of greater than about 0.70. A charge carrier separation layer can be coupled adjacent and between the diamond-like carbon cathode and an anode. The conductive diamond-like carbon material of the present invention can be useful for any other application which can benefit from the use of conductive and transparent electrodes which are also chemically inert, radiation damage resistance, and are simple to manufacture.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/111,052, filed on May 19, 2011, now issued as U.S. Pat. No.8,227,812, which is a continuation of U.S. patent application Ser. No.12/826,502, filed on Jun. 29, 2010, now issued as U.S. Pat. No.7,951,642, which is a divisional of U.S. patent application Ser. No.11/893,589, filed on Aug. 14, 2007, now issued as U.S. Pat. No.7,745,831, which claims the benefit of U.S. Provisional Application No.60/837,885, filed Aug. 14, 2006, each of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to diamond-like carbonmaterials, and to devices and methods that utilize conductivediamond-like carbon material. Accordingly, the present applicationinvolves the fields of physics, chemistry, electricity, and materialscience.

BACKGROUND OF THE INVENTION

Solar cell technologies have progressed over the past several decadesresulting in significant contributions to potential power sources inmany different applications. Despite dramatic improvements in materialsand manufacturing methods, solar cells still have efficiency limits wellbelow theoretical efficiencies, with current conventional solar cellshaving maximum efficiency of around 26%. Various approaches haveattempted to increase efficiencies with some success. For example, priorapproaches have included light trapping structures and buried electrodesin order to minimize surface area shaded by the conductive metal grid.Other methods have included a rear contact configuration whererecombination of hole-electron pairs occurs along the rear side of thecell.

However, these and other approaches still suffer from drawbacks such asmediocre efficiencies, manufacturing complexities, material costs,reliability, and radiation degradation, among others. As such, materialscapable of achieving high current outputs by absorbing relatively lowamounts of energy from an energy source, and which are suitable for usein practical applications continue to be sought through ongoing researchand development efforts.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides materials, devices, andmethods for enhancing performance of electronic devices such as solarcells, thermoelectric conversion devices and other electronic devices.In accordance with the present invention, a diamond-like carbonelectronic device can include a diamond-like carbon cathode. The cathodecan comprise a conductive diamond-like carbon material having an sp³bonded carbon content from about 30 atom % to about 90 atom %, ahydrogen content from 0 atom % to about 30 atom %, and an sp² bondedcarbon content from about 10 atom % to about 70 atom %. A charge carrierseparation layer can be coupled adjacent the diamond-like carboncathode. Further, an anode can be adjacent the charge carrier separationlayer opposite the diamond-like carbon cathode.

In another alternative aspect of the present invention, a conductivediamond-like carbon material can comprise a diamond-like carbon materialhaving a resistivity from about 0 μΩ-cm to about 80 μΩ-cm at 20° C. suchthat the material is electrically conductive. Further, the diamond-likecarbon material can have a visible light transmissivity from about 0.5to about 1.0. The conductivity and visible light transmissivity can be afunction of sp² and sp³ bonded carbon content, hydrogen content, andoptional conductive additives. For example, an increase in sp² bondedcarbon content can increase conductivity while decreasingtransmissivity. Conversely, an increase in hydrogen content and/or sp³bonded carbon content can lead to increases in transmissivity anddecrease in conductivity. Conductivity and transmissivity can also beaffected by introduction of additives such as dopants or conductivematerials.

The conductive diamond-like carbon material of the present invention canbe useful for a variety of applications such as, but not limited to,solar cell electrodes, fuel cells, LED electrodes, thermoelectricconversion devices, or any other application which can benefit from theuse of conductive and transparent electrodes which are also chemicallyinert, radiation damage resistance, and are simple to manufacture.

In one detailed aspect of the present invention, the charge carrierseparation layer can be a semiconductor layer to form a solar cell. Thediamond-like carbon cathode of the present invention can allow for thesemiconductor layer to be substantially planar. For example, the solarcell can be free of trenches and/or metal grid materials which arecommonly present in conventional silicon solar cells. Further, thecharge carrier separation layer can have a thickness from about 10 μm toabout 300 μm.

In yet another detailed aspect of the present invention, the chargecarrier separation layer can form a multi-junction solar cell.Alternatively, the charge carrier separation layer can include acompositionally graded material including carbon and semi-conductor.

The present invention further includes methods of forming a diamond-likecarbon electronic device. A charge carrier separation layer can beprepared having desired properties and characteristics designed for aparticular device. An anode can be formed adjacent the charge carrierseparation layer. Further, a diamond-like carbon cathode can be formedand coupled to the charge carrier separation layer opposite the anode.Based on the following detailed discussion, the diamond-like carboncathode can include a diamond-like carbon material having a conductivityfrom about 0 μΩ-cm to about 80 μΩ-cm at 20° C. and a visible lighttransmissivity from about 0.5 to about 1.0.

There has thus been outlined, rather broadly, the more importantfeatures of the invention so that the detailed description thereof thatfollows may be better understood, and so that the present contributionto the art may be better appreciated. Other features of the presentinvention will become clearer from the following detailed description ofthe invention, taken with the accompanying drawings and claims, or maybe learned by the practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side cross-sectional view of a conventional silicon solarcell in accordance with the prior art.

FIG. 2 shows a side cross-sectional view of a diamond-like carboncomposite solar cell in accordance with one embodiment of the presentinvention.

FIG. 3 shows a side cross-sectional view of a compositionally gradeddiamond-like carbon composite solar cell in accordance with anotherembodiment of the present invention.

FIG. 4 shows an SEM graph of amorphous diamond material illustrating thevariation in asperity size and shape.

FIG. 5 is a micrograph of a mosaic surface of a diamond-like carbonmaterial grown heteroepitaxially on a silicon substrate in accordancewith one aspect of the present invention.

The drawings will be described further in connection with the followingdetailed description. Further, these drawings are not necessarily toscale and are by way of illustration only such that dimensions andgeometries can vary from those illustrated.

DETAILED DESCRIPTION

Before the present invention is disclosed and described, it is to beunderstood that this invention is not limited to the particularstructures, process steps, or materials disclosed herein, but isextended to equivalents thereof as would be recognized by thoseordinarily skilled in the relevant arts. It should also be understoodthat terminology employed herein is used for the purpose of describingparticular embodiments only and is not intended to be limiting.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a layer” includes one or more of such layers, reference to“an additive” includes reference to one or more of such materials, andreference to “a cathodic arc technique” includes reference to one ormore of such techniques.

Definitions

In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set forthbelow.

As used herein, “charge carrier separation layer” refers to any materialor layer which provides an electric potential barrier to free electronflow. Non-limiting examples of charge carrier separation layers caninclude p-n junctions, p-i-n junctions, electrolyte solutions, thin filmjunctions (e.g. thin dielectric films), and the like.

As used herein, “electrode” refers to a conductor used to makeelectrical contact between at least two points in a circuit.

As used herein, “sp³ bonded carbon” refers to carbon atoms bonded toneighboring carbon atoms in a crystal structure substantiallycorresponding to the diamond isotope of carbon (i.e. pure sp³ bonding),and further encompasses carbon atoms arranged in a distorted tetrahedralcoordination sp³ bonding, such as amorphous diamond and diamond-likecarbon.

As used herein, “sp² bonded carbon” refers to carbon atoms bonded toneighboring carbon atoms in a crystal structure substantiallycorresponding to the graphitic isotope of carbon.

As used herein, “diamond” refers to a crystalline structure of carbonatoms bonded to other carbon atoms in a lattice of tetrahedralcoordination known as sp³ bonding. Specifically, each carbon atom issurrounded by and bonded to four other carbon atoms, each located on thetip of a regular tetrahedron. Further, the bond length between any twocarbon atoms is 1.54 angstroms at ambient temperature conditions, andthe angle between any two bonds is 109 degrees, 28 minutes, and 16seconds although experimental results may vary slightly. The structureand nature of diamond, including many of its physical and electricalproperties are well known in the art.

As used herein, “distorted tetrahedral coordination” refers to atetrahedral bonding configuration of carbon atoms that is irregular, orhas deviated from the normal tetrahedron configuration of diamond asdescribed above. Such distortion generally results in lengthening ofsome bonds and shortening of others, as well as the variation of thebond angles between the bonds. Additionally, the distortion of thetetrahedron alters the characteristics and properties of the carbon toeffectively lie between the characteristics of carbon bonded in sp³configuration (i.e. diamond) and carbon bonded in sp² configuration(i.e. graphite). One example of material having carbon atoms bonded indistorted tetrahedral bonding is amorphous diamond. It will beunderstood that many possible distorted tetrahedral configurations existand a wide variety of distorted configurations are generally present inamorphous diamond.

As used herein, “diamond-like carbon” refers to a carbonaceous materialhaving carbon atoms as the majority element, with a substantial amountof such carbon atoms bonded in distorted tetrahedral coordination.Diamond-like carbon (DLC) can typically be formed by PVD processes,although CVD or other processes could be used such as vapor depositionprocesses. Notably, a variety of other elements can be included in theDLC material as either impurities, or as dopants, including withoutlimitation, hydrogen, sulfur, phosphorous, boron, nitrogen, silicon,tungsten, etc.

As used herein, “amorphous diamond” refers to a type of diamond-likecarbon having carbon atoms as the majority element, with a substantialamount of such carbon atoms bonded in distorted tetrahedralcoordination. In one aspect, the amount of carbon in the amorphousdiamond can be at least about 90%, with at least about 20% of suchcarbon being bonded in distorted tetrahedral coordination. Amorphousdiamond also has a higher atomic density than that of diamond (176atoms/cm³). Further, amorphous diamond and diamond materials contractupon melting.

As used herein, “transmissivity” refers to the portion of light whichtravels across a material. Transmissivity is defined as the ratio of thetransmitted light intensity to the total incident light intensity andcan range from 0 to 1.0.

As used herein, “vapor deposited” refers to materials which are formedusing vapor deposition techniques. “Vapor deposition” refers to aprocess of depositing materials on a substrate through the vapor phase.Vapor deposition processes can include any process such as, but notlimited to, chemical vapor deposition (CVD) and physical vapordeposition (PVD). A wide variety of variations of each vapor depositionmethod can be performed by those skilled in the art. Examples of vapordeposition methods include hot filament CVD, rf-CVD, laser CVD (LCVD),laser ablation, conformal diamond coating processes, metal-organic CVD(MOCVD), sputtering, thermal evaporation PVD, ionized metal PVD (IMPVD),electron beam PVD (EBPVD), reactive PVD, atomic layer deposition (ALD),and the like.

As used herein, “asperity” refers to the roughness of a surface asassessed by various characteristics of the surface anatomy. Variousmeasurements may be used as an indicator of surface asperity, such asthe height of peaks or projections thereon, and the depth of valleys orconcavities depressing therein. Further, measures of asperity includethe number of peaks or valleys within a given area of the surface (i.e.peak or valley density), and the distance between such peaks or valleys.

As used herein, “metallic” refers to a metal, or an alloy of two or moremetals. A wide variety of metallic materials are known to those skilledin the art, such as aluminum, copper, chromium, iron, steel, stainlesssteel, titanium, tungsten, zinc, zirconium, molybdenum, etc., includingalloys and compounds thereof

As used herein, “dielectric” refers to any material which iselectrically resistive. Dielectric materials can include any number oftypes of materials such as, but not limited to, glass, polymers,ceramics, graphites, alkaline and alkali earth metal salts, andcombinations or composites thereof

As used herein, “vacuum” refers to a pressure condition of less than10⁻² torr.

As used herein, “electrically coupled” refers to a relationship betweenstructures that allows electrical current to flow at least partiallybetween them. This definition is intended to include aspects where thestructures are in physical contact and those aspects where thestructures are not in physical contact. Typically, two materials whichare electrically coupled can have an electrical potential or actualcurrent between the two materials. For example, two plates physicallyconnected together by a resistor are in physical contact, and thus allowelectrical current to flow between them. Conversely, two platesseparated by a dielectric material are not in physical contact, but,when connected to an alternating current source, allow electricalcurrent to flow between them by capacitive means. Moreover, depending onthe insulative nature of the dielectric material, electrons may beallowed to bore through, or jump across the dielectric material whenenough energy is applied.

As used herein, “adjacent” refers to near or close sufficient to achievea desired affect. Although direct physical contact is most common andpreferred in the layers of the present invention, adjacent can broadlyallow for spaced apart features.

As used herein, “thermoelectric conversion” relates to the conversion ofthermal energy to electrical energy or of electrical energy to thermalenergy, or flow of thermal energy.

As used herein, the term “substantially” refers to the complete ornearly complete extent or degree of an action, characteristic, property,state, structure, item, or result. The exact allowable degree ofdeviation from absolute completeness may in some cases depend on thespecific context. However, generally speaking the nearness of completionwill be so as to have the same overall result as if absolute and totalcompletion were obtained. The use of “substantially” is equallyapplicable when used in a negative connotation to refer to the completeor near complete lack of an action, characteristic, property, state,structure, item, or result. For example, a composition that is“substantially free of” particles would either completely lackparticles, or so nearly completely lack particles that the effect wouldbe the same as if it completely lacked particles. In other words, acomposition that is “substantially free of” an ingredient or element maystill actually contain such item as long as there is no measurableeffect on the property of interest thereof.

As used herein, the term “about” is used to provide flexibility to anumerical range endpoint by providing that a given value may be “alittle above” or “a little below” the endpoint with a degree offlexibility as would be generally recognized by those skilled in theart. Further, the term about explicitly includes the exact endpoint,unless specifically stated otherwise.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary.

Concentrations, amounts, and other numerical data may be expressed orpresented herein in a range format. It is to be understood that such arange format is used merely for convenience and brevity and thus shouldbe interpreted flexibly to include not only the numerical valuesexplicitly recited as the limits of the range, but also to include allthe individual numerical values or sub-ranges encompassed within thatrange as if each numerical value and sub-range is explicitly recited. Asan illustration, a numerical range of “about 1 to about 5” should beinterpreted to include not only the explicitly recited values of about 1to about 5, but also include individual values and sub-ranges within theindicated range. Thus, included in this numerical range are individualvalues such as 2, 3, and 4 and sub-ranges such as from 1-3, from 2-4,and from 3-5, etc., as well as 1, 2, 3, 4, and 5, individually. Thissame principle applies to ranges reciting only one numerical value as aminimum or a maximum Furthermore, such an interpretation should applyregardless of the breadth of the range or the characteristics beingdescribed.

The Invention

FIG. 1 illustrates a conventional crystalline silicon solar cell 10 inaccordance with the prior art. An anti-reflection layer 12 is used toprevent excessive reflection of light from the surface of the underlyingsilicon layer 14. The anti-reflection layer is most often siliconnitride, although other materials have also been used, and iselectrically insulating. The silicon solar cell shown in FIG. 1 iscommonly referred to as a p-i-n solar cell due to the respective n and pdoping of cathode area 16 and anode area 18 on either side of aninsulating inner layer 20. A conductive metal grid 22 is buried into thesilicon layer. The metal grid is typically formed of silver which issintered at about 800° C. or other conductive metals like copper ornickel. The metal grid is embedded a significant distance into theinsulating layer, e.g. typically over about 400 to 500 nm. A conductivemetal such as silver or other suitable material is also used as theanode 24. Although many other considerations are important in designingsolar cells, such are well within the knowledge of one skilled in theart. Further, the above description provides a suitable background forthe following discussion of various aspects of the present invention andthe contribution thereof to the art.

In one aspect of the present invention, a diamond-like carbon electronicdevice may include a conductive diamond-like carbon cathode, a chargecarrier separation layer adjacent the diamond-like carbon cathode, andan anode adjacent the charge carrier separation layer opposite thediamond-like carbon cathode. Another aspect of the present invention caninclude a diamond-like carbon material having a resistivity from about 0μΩ-cm to about 80 μΩ-cm at 20° C. such that the material is electricallyconductive. In another aspect the resistivity of the conductivediamond-like carbon material can be from about 0 μΩ-cm to about 40μΩ-cm. Further, the diamond-like carbon material can have a visiblelight transmissivity from about 0.5 to about 1.0. The conductivity andvisible light transmissivity can be a function of sp² and sp³ bondedcarbon content, hydrogen content, and optional conductive additives. Forexample, an increase in sp² bonded carbon content can increaseconductivity while decreasing transmissivity. Conversely, an increase inhydrogen content and/or sp³ bonded carbon content can lead to increasesin transmissivity and decrease in conductivity. Conductivity andtransmissivity can also be affected by introduction of additives such asdopants or conductive materials.

The conductive diamond-like carbon material of the present invention canbe useful for a variety of applications such as, but not limited to,solar cell electrodes, LED electrodes, or other applications whereconductive and transparent electrodes which are also chemically inert,radiation damage resistant, and are simple to manufacture.

In accordance with one embodiment of the present invention, adiamond-like carbon electronic device can include a diamond-like carboncathode. The cathode can comprise a conductive diamond-like carbonmaterial having an sp^(a) bonded carbon content from about 30 atom % toabout 90 atom %, a hydrogen content from 0 atom % to about 30 atom %,and an sp² bonded carbon content from about 10 atom % to about 70 atom%. A charge carrier separation layer can be coupled adjacent thediamond-like carbon cathode. Further, an anode can be adjacent thecharge carrier separation layer opposite the diamond-like carboncathode. It should be noted that the terms “cathode” and “anode” areintended to be interchangeable, and are used to merely signify adifference in polarity between two electrodes. For example, applicationsusing alternating current would involve electrodes which serve as bothanode and cathode.

Referring now to FIG. 2, the diamond-like carbon electronic device canbe a solar cell 26. In this embodiment of the present invention, thecharge carrier separation layer can be a semiconductor layer 28 adjacentan anode 36. The semiconductor layer can be formed of any suitablematerial such as, but not limited to, crystalline silicon, amorphoussilicon, gallium arsenide, gallium indium phosphide, gallium indiumnitride, copper indium diselenide, cadmium telluride, and composites orcombinations thereof In one embodiment of the present invention, thecharge carrier separation layer can comprise crystalline silicon. Insome cases, the charge carrier separation layer can consist essentiallyof crystalline silicon, with the exception of suitable dopants. Inanother preferred embodiment, the charge carrier separation layer cancomprise or consist essentially of amorphous silicon, again with theexception of suitable dopants. Crystalline-based charge carrierseparation layers can be particularly used in formation of solar cells.In contrast, a partially or fully amorphous charge separation carrierlayer can be particularly useful in thermoelectric conversion devices,e.g. those which convert heat into electricity.

Suitable charge separation carrier layers can be formed using a varietyof methods such as, but not limited to, vapor deposition, epitaxialgrowth or the like. Most often, a starting material such as aconventional silicon wafer can be used from conventional commercialsources. The wafer can be cut from a solid silicon ingot or boule. Thewafer can be polished to have a smooth surface. Alternatively, thesemiconductor material can be formed directly on a desired substrate oranode using vapor deposition or other suitable techniques. Further, thewafer or semiconductor surface can be etched to roughen the surfaceand/or may include features such as pyramidal depressions or extensionswhich increase functional surface areas of the device. These featurescan be varied in size and geometry based on balancing performance, cost,and material limitations. For example, etching of pyramidal extensionsor depressions can be particularly useful in enhancing performance. Suchextension and depressions are most often from about 0.01 mm to about 0.7mm, although other dimensions can be useful.

Generally, the present invention also provides methods for formingdiamond-like carbon electronic devices. Such a method may includeforming an anode on a suitable charge carrier separation layer andcoupling a conductive diamond-like carbon cathode to the charge carrierseparation layer opposite the anode. The conductive diamond-like carboncathode may be formed on the charge carrier separation layer oppositethe anode, or it may be formed separately therefrom and subsequentlycoupled thereto as described in more detail throughout. Additionally, inone aspect the anode may be a conductive diamond-like carbon anode.

The diamond-like carbon electronic devices of the present inventionallow for a significant reduction in the thickness of the charge carrierseparation layer. At least one reason for this is the elimination orreduction of any buried metal grid electrodes. The diamond-like carboncathode of the present invention can allow for the semiconductor layerto be substantially planar. For example, the devices of the presentinvention can be free of trenches and/or metal grid materials which arepresent in conventional silicon solar cells. As a general guideline, thecharge carrier separation layer of the present invention can have athickness from about 10 μm to about 300 μm, and preferably from about 50μm to about 150 μm, with about 100 μm being currently most preferred.The use of conductive diamond-like carbon material can also preventthese thinner semiconductor layers from warping.

The charge carrier separation layer can most often be configured as ap-i-n or p-n junction. For example, FIG. 2 illustrates a p-i-n junctionwhere the charge carrier separation layer comprises a semiconductorlayer which is doped on each side with either p- or n-doping materials.An n-doped area 30 can be formed by conventional doping of thesemiconductor layer 28. Suitable n-dopants can include, but are notlimited to, phosphorous, arsenic, bismuth, antimony, and combinationsthereof Similarly, a p-doped area 32 can be formed by doping of thesemiconductor layer with dopants such as, but not limited to, boron,aluminum, gallium, indium, thallium, and combinations thereof The degreeof doping can be controlled by the conditions during doping such asdopant concentration, temperature, and the like. The charge carrierseparation layer can alternatively be formed of distinct layers in a p-nconfiguration. Specifically, an n-doped semiconductor layer can beformed adjacent a p-doped semiconductor layer to form the charge carrierseparation layer. The semiconductor materials can be selectively dopedusing methods such as, but not limited to, ion implantation, drive-indiffusion, field-effect doping, electrochemical doping, vapordeposition, or the like. Further, such doping can be accomplished byco-deposition with the semiconductor material. For example, a boronsource gas and silicon or other semiconductor source gas can besimultaneously present in a vapor deposition chamber.

The diamond-like carbon electronic device of the present invention caninclude a cathode 34 comprising a conductive diamond-like carbonmaterial. The conductive diamond-like carbon material of the presentinvention represents a distinct class of diamond-like carbon materialshaving the properties identified herein such as low resistivity, hightransmissivity, etc. As mentioned earlier, these properties are at leastpartially related to variables such as hydrogen content, sp² and sp^(a)bonded carbon content, and optional additives or dopants. Most often,the conductive diamond-like material can be formed on the semiconductorlayer by a suitable vapor deposition process. Additionally, in onespecific embodiment, the diamond-like carbon material may be amorphousdiamond.

Increased hydrogen content can contribute to an increase intransmissivity. The hydrogen content can be incorporated throughout theconductive diamond-like material or substantially only at a surfacethereof In one embodiment, any hydrogen content can be substantiallyonly at external surfaces of the conductive diamond-like material. As ageneral guideline, in one specific embodiment the hydrogen content canrange from 0 atom % to about 30 atom %. In another specific embodimentthe hydrogen content can range from about 15 atom % to about 25 atom %.In one alternative embodiment, the conductive diamond-like carbon can besubstantially free of hydrogen content. However, hydrogen content can beincreased by increasing hydrogen gas concentrations during deposition ofthe diamond-like carbon material. Alternatively, a diamond-like carbonmaterial can be heat treated with hydrogen gas to form a hydrogenterminated surface layer of diamond-like carbon. Typically, depositionoccurs using a vapor deposition process such as chemical vapordeposition, although other methods can be suitable.

Increased hydrogen content can also be accompanied by decreasedconductivity, all other variables being factored out. Therefore, it cansometimes be desirable to introduce a conductive additive in relativelysmall amounts to increased conductivity. Suitable conductive additivescan be incorporated via any number of approaches such as, but notlimited to, codeposition in the vapor phase, coating of the conductiveadditives by the diamond-like carbon material, infiltration, alternatingdeposition of each material, and the like. Suitable conductive additivescan include conductive metal particulates incorporated into ahydrogenated diamond-like carbon material. Suitable metal particles cancomprise metals such as silver, copper, gold, titanium, or other similarmaterials. In one specific aspect, the metal particles can benanoparticles, i.e. 100 nm or less and often 50 nm or less. Otherconductive additives can include, but are not limited to, carbonnanotubes, graphite, and the like. In one particular scenario, theconductive additive can be a graphite having a degree of graphitizationgreater than 0.8 and often greater than 0.9. Further, in order to avoidexcessive decrease in transmissivity due to the conductive additives thesize and/or the concentration of the additives can be decreased. Suchparticulates can be any suitable size, although about 1 nm to about 1 μmis typically suitable with about 2 nm to about 100 nm being suitable andabout 0.1 μm to about 0.6 μm being preferred. Smaller particle sizesallow for increased transmissivity but also experience a contrastingdecrease in contribution to conductivity of the diamond-like material.Similarly, concentration of metal additives can generally range fromabout 2 vol % to about 60 vol %, although optimal particle sizes andconcentrations can vary considerably depending on the specific particlematerial, sp² and sp^(a) bonded carbon content, and hydrogen content.These same principles can be equally applied to the anode side of thedevice.

Another important variable with respect to the conductive diamond-likecarbon material is the sp³ bonded carbon content. Advantageously, anincrease in sp³ bonded carbon content results in an increase intransmissivity as the diamond character of the material increase.However, this is also accompanied by an associated decrease inconductivity. Generally, the conductive diamond-like carbon material,whether at the cathode and/or anode can have an sp³ bonded carboncontent from about 30 atom % to about 90 atom %. At higher sp³ bondedcarbon contents, e.g. from about 50 atom % to about 90 atom %,additional additives and/or dopants can be introduced to increaseconductivity sufficient for use of the material as a conductiveelectrode within the device. For example, doping with nitrogen or othersimilar dopants can provide good results without significantlydecreasing transmissivity in the case of use in the cathode.

Further, sp² bonded carbon content can also contribute to decreasedtransmissivity. However, sp² bonded carbon is graphitic in crystalstructure and is electrically conductive. Therefore, an appropriatebalance of sp² bonded carbon content should be considered. As a generalguideline, in one embodiment the conductive diamond-like carbon materialcan have from about 10 atom % to about 70 atom % sp² bonded carboncontent. In another embodiment, the conductive diamond-like carbonmaterial can have from about 35 atom % to about 60 atom %. However, thespecific content can depend on the hydrogen content, sp³ bonded carboncontent, and other optional additives and/or dopants. However, the sp²bonded carbon content can preferably be sufficient to provide theconductive diamond-like carbon material with a visible lighttransmissivity of greater than about 0.70, and most preferably greaterthan about 0.90.

The diamond-like carbon material can be made using any suitable method,such as various vapor deposition processes. In one aspect of theinvention, the diamond-like carbon material can be formed using acathodic arc method. Various cathodic arc processes are well known tothose of ordinary skill in the art, such as those disclosed in U.S. Pat.Nos. 4,448,799; 4,511,593; 4,556,471; 4,620,913; 4,622,452; 5,294,322;5,458,754; and 6,139,964, each of which is incorporated herein byreference. Generally speaking, cathodic arc techniques involve thephysical vapor deposition (PVD) of carbon atoms onto a target, orsubstrate. The arc is generated by passing a large current through agraphite electrode that serves as an anode, and vaporizing carbon atomswith the current. If the carbon atoms contain a sufficient amount ofenergy (i.e. about 100 eV) they will impinge on the target and adhere toits surface to form a carbonaceous material, such as amorphous diamond.Amorphous diamond can be coated on almost any metallic substrate,typically with no, or substantially reduced, contact resistance. Ingeneral, the kinetic energy of the impinging carbon atoms can beadjusted by the varying the negative bias at the substrate and thedeposition rate can be controlled by the arc current. Control of theseparameters as well as others can also adjust the degree of distortion ofthe carbon atom tetrahedral coordination and the geometry, orconfiguration of the amorphous diamond material (i.e. for example, ahigh negative bias can accelerate carbon atoms and increase sp^(a)bonding). By measuring the Raman spectra of the material the sp^(a)/sp²ratio can be determined However, it should be kept in mind that thedistorted tetrahedral portions of the amorphous diamond layer aregenerally neither pure sp^(a) nor sp² but a range of bonds which are ofintermediate character. Further, increasing the arc current can increasethe rate of target bombardment with high flux carbon ions. As a result,temperature can rise so that the deposited carbon will convert to morestable graphite. Thus, final configuration and composition (i.e. bandgaps, NEA, and emission surface asperity) of the diamond-like carbonmaterial can be controlled by manipulating the cathodic arc conditionsunder which the material is formed.

Additionally, other processes can be used to form diamond-like carbonsuch as various vapor deposition processes, e.g. chemical vapordeposition or the like. In preparing more crystalline diamond-likecarbon, chemical vapor deposition can be used. Chemical vapor deposition(CVD) of diamond-like carbon can generally be performed by introducing acarbon source gas at elevated temperatures into a chamber housing adeposition substrate, e.g. semiconductor or charge separation carrierlayer. Diamond-like carbon is typically deposited using physical vapordeposition (PVD) which involves impinging carbon atoms against asubstrate at relatively low deposition and plasma temperatures (e.g.100° C.). Due to the low temperature of such PVD methods, carbon atomsare not located at thermal equilibrium positions. As a result, the filmcan be less stable with high internal stress. Alternatively, CVDprocesses can be employed to deposit diamond-like carbon. If thedeposition temperature is high (e.g. 800° C.), diamond will grow tobecome a crystalline CVD diamond film An example of a suitable CVDprocess is radio frequency (13.6 MHz) CVD by dissociation of acetylene(C₂H₂) and hydrogen gas under partial vacuum (millitorr). Alternatively,pulsed DC can be used in stead of RF CVD. In the case of amorphousdiamond, deposition by cathodic arc or laser ablation can form asuitable layer. Conductive diamond-like carbon material can be used asthe cathode and optionally also as the anode of devices of the presentinvention.

The formation of the cathode and/or anode may be further facilitatedthrough the deposition of a conformal diamond-like carbon layer.Conformal diamond coating processes can provide a number of advantagesover conventional diamond film processes. Conformal diamond coating canbe performed on a wide variety of substrates, including non-planarsubstrates. A growth surface can be pretreated under diamond growthconditions in the absence of a bias to form a carbon film The diamondgrowth conditions can be conditions which are conventional vapordeposition conditions for diamond without an applied bias. As a result,a thin carbon film can be formed which is typically less than about 100angstroms. The pretreatment step can be performed at almost any growthtemperature such as from about 200° C. to about 900° C., although lowertemperatures below about 500° C. may be preferred. Without being boundto any particular theory, the thin carbon film appears to form within ashort time, e.g., less than one hour, and is a hydrogen terminatedamorphous carbon.

Following formation of the thin carbon film, the growth surface may thenbe subjected to diamond growth conditions to form the diamond-likecarbon layer. The diamond growth conditions may be those conditionswhich are commonly used in traditional vapor deposition diamond growth.However, unlike conventional amorphous diamond film growth, an amorphousdiamond film produced using the above pretreatment steps results in aconformal amorphous diamond film that typically begins growthsubstantially over the entire growth surface with substantially noincubation time.

One aspect of the diamond-like carbon material that facilitates electronemission is the distorted tetrahedral coordination with which many ofthe carbon atoms are bonded. Tetrahedral coordination allows carbonatoms to retain the sp^(a) bonding characteristic that provides aplurality of effective band gaps, due to the differing bond lengths ofthe carbon atom bonds in the distorted tetrahedral configuration.

In one aspect of the present invention, the upper exposed surface of theelectronic device can be configured to improve energy absorption.Specifically, as shown in FIG. 2, the surface area of the outer layercan be increased by forming features which extend outwardly such as thepyramids shown. However, other shaped features can be suitable for usein the present invention. This not only increases surface area forexposure to light or other energy sources, but also provides anincreased junction surface area per total area of the device. Further,the diamond-like carbon material can have a surface roughness whichfurther increases the surface area on a much smaller scale thanillustrated and than the pyramid features. FIG. 4 is a micrograph of adiamond-like carbon material suitable for use in the present inventionillustrating random asperities of typical amorphous diamond. Thefeatures can typically have dimensions in the tens of microns range,while the diamond-like carbon material can have asperities in thenanometer range. In one aspect, the diamond-like carbon material canhave an average surface asperity having a height of from about 10 toabout 10,000 nanometers. In another aspect, the diamond-like carbonmaterial can have an average asperity height of from about 10 to about1,000 nanometers. In yet another aspect, the average asperity height canbe about 800 nanometers. In a further aspect, the average asperityheight can be about 100 nanometers. Further, in one aspect the asperitycan have an average peak density of at least about 1 million peaks persquare centimeter of emission surface. In another aspect, the averagepeak density can be at least about 100 million peaks per squarecentimeter of the surface. In yet another aspect, the peak density canbe at least about 1 billion peaks per square centimeter of the surface.In a further aspect, the average asperity can include a height of about800 nanometers and a peak density of at least about, or greater thanabout 1 million peaks per square centimeter of emission surface. In yeta further aspect, the asperity can include a height of about 1,000nanometers and a peak density of at least about, or greater than 1billion peaks per square centimeter of the surface.

Further, the diamond-like carbon material can be grown as apolycrystalline film on a silicon substrate, preferably the (100) faceof a silicon wafer. By orienting the nuclei, formingcrystallographically oriented pits, and/or minimizing the growth rate,the multiple grains can merge to form a mosaic structure having similarorientations such that the overall crystal structure is intermediatebetween single crystalline, which can be difficult to achieve, andpolycrystalline. FIG. 5 is a micrograph of such a mosaic structure ofdiamond-like carbon grown hetero-epitaxially on a 100 face of silicon.

As noted above, various dopants can enhance or control the conductivityof the conductive diamond-like carbon materials of the presentinvention. Most often suitable dopants can include B, N, Si, P, Li,conductive metal, or combinations thereof, although other materials canalso be effective. Boron doped diamond-like carbon materials can behighly transmissive and is also conductive. For example, hydrogenatedboron doped diamond-like carbon material can be formed using a radiofrequency chemical vapor deposition process including a carbon sourcegas and a boron source gas. Non-limiting examples of carbon sourcesgases can include methane and ethylene. Similarly, non-limiting examplesof boron source gases can include BH₃ and B₂H₂, although other borongases can be used. This hydrogenated boron doped diamond material andother diamond-like materials of the present invention can serve as aconductive layer, passivation layer, and an anti-reflection layer. Thus,a conventional SiN anti-reflection layer can be eliminated Similarly,passivation steps and techniques can be reduced or entirely eliminated.The diamond-like carbon material can enhance resistance to mechanicalscratching and chemical corrosion. Further, an optional metal materialcan be added during formation of the boron doped diamond-like carbonmaterial. For example, a silver or other metal vapor can also beintroduced during vapor deposition.

Additionally, excessive grain boundaries within the conductivediamond-like carbon material can increase resistivity of the material.Grain boundaries can be minimized using a variety of techniques. In onecurrently preferred approach, formation of the diamond-like carbonmaterial can be initiated using nanodiamond seeding. Alternatively, hightemperature annealing of the diamond-like carbon material can help toreduce grain boundaries somewhat. Further, surface roughness of theconductive diamond-like carbon material can be less than about 30 nmThis can be accomplished using a suitable technique. For example,reverse casting using a smooth inverse mold, polishing, or other similarprocesses can be used. In yet another alternative aspect of the presentinvention, the semiconductor layer can be prepared by etching to roughenthe n-doped surface of the semiconductor. This can enhance diamond-likecarbon grain growth and further increase electron flow into thediamond-like carbon material layer.

In accordance with the present invention, an electronic device caninclude a diamond-like carbon electronic cathode which consistsessentially of the conductive diamond-like carbon. This is particularlyuseful in the context of forming solar cells. In this manner, the solarcell cathode can exclude any metal grids or other materials whichcontribute to decreases in transmissivity. Conventional metal leads canbe formed around the periphery of the diamond-like carbon cathode whichcan be used to integrate the electronic device as part of a completecircuit. Alternatively, or in addition, the cathode can be formed bydepositing diamond-like carbon on a metallic layer such as silver greaseor other suitable conductive layer.

The conductive diamond-like carbon cathode or anode of the presentinvention can have any functional thickness. However, the cathode cantypically have a thickness from about 0.01 μm to about 10 μm. Thus, theoverall device can, in some embodiments, measure from about 10 μm toabout 350 μm, and in some cases from about 50 μm to about 150 μm.

Further, the anode can be formed of any suitable conductive material.Non-limiting examples of suitable conductive materials can includesilver, gold, tin, copper, aluminum, and alloys thereof Alternatively,at least a portion of the anode can be formed of a conductivediamond-like carbon material. Although the same parameters can be usedas described above, transmissivity of the anode is often less important.Therefore, a higher sp² carbon bonded content can be tolerated than forthe cathode side without the need for additives or dopants.

In another embodiment of the present invention, the charge carrierseparation layer can serve as one or more portions of a p-n or p-i-njunction. For example, the charge carrier separation layer can bep-doped while the conductive diamond-like carbon cathode can be n-dopedin a region adjacent the charge carrier separation layer, as previouslydiscussed. Thus, the charge carrier separation layer would serve only asa single portion of the junction. Similarly, the conductive diamond-likecarbon cathode can include multiple regions, e.g. a conductive electroderegion remote or opposite from the charge carrier separation layer andan n-doped region proximate to the charge carrier separation layer.

In yet another detailed aspect of the present invention, the chargecarrier separation layer can form a multi-junction solar cell. Multiplejunctions can be configured having a variation in bandgaps. Typically, asingle junction is capable of absorbing light corresponding to aspecific bandgap for the materials comprising the junction. By preparingand configuring multiple junctions in series across the device eachjunction can have a different bandgap. As a result, a larger percentageof incoming energy can be converted to useful work, e.g. electricity.The higher bandgap materials can be placed above, i.e. closer to thecathode and light entry side, the lower bandgap materials. The chargecarrier separation layer can be multiple p-n and/or p-i-n junctions toform a multi junction solar cell. The bandgap of each layer can beadjusted by varying dopant concentration, type and/or semiconductormaterial, e.g. silicon, gallium-based materials, or the like. In orderto fully take advantage of the multiple junctions, the thickness isoften maintained sufficiently thin to allow at least some light to passthrough each layer and the layers can be formed of light transmissivematerials as discussed elsewhere.

Alternatively, the charge carrier separation layer can include acompositionally graded material including carbon and semi-conductor.This approach has the added advantage of reducing thermal mismatchstresses between adjacent layers or by eliminating distinct layersaltogether. The charge carrier separation layer can comprise acompositionally graded material of carbon and a semi-conductor. Forexample, the charge carrier separation layer can be graded from pure Sito SiC (e.g. in distinct layers of 10/20/30/40/50% Si or by a continuousgradation of materials) with a SiC layer adjacent the conductivediamond-like carbon material of the cathode. Optionally, each layer canbe progressively doped having higher bandgaps with higher carboncontent. Compositional differences can be achieved, for example, byvarying gas source concentrations or by varying impact energy oncorresponding sputtering targets.

In one aspect of the present invention, the graded material can includeat least four distinct compositionally graded layers. Typically, fromfour to about ten graded layers and preferably four to about six layerscan be formed. Alternatively, full spectrum solar cells utilizing indiumgallium nitride may also be suitable in connection with the presentinvention.

As an additional benefit of the present invention, each of the cathode,anode and charge carrier separation layers can be formed or prepared ata temperature below about 750° C., and preferably below about 650° C.Such low temperature processing can prevent or significantly reducewarpage. Additionally, amorphous diamond has a high radiation hardnesssuch that it is resistant to aging and degradation over time. Incontrast, typical semiconductor materials are UV degradable and tend tobecome less reliable over time. The use of amorphous or diamond-likecarbon material has the further advantage of reducing thermal mismatchbetween layers of the device. For example, silicon, silicon carbide anddiamond-like carbon have a thermal expansion coefficient of around 4ppm/° C. (near the process temperatures used herein) which dramaticallyreduces thermal mismatch stress during and after processing. This affectalso reduces delamination as substantially all of the layers of thedevice can have substantially similar thermal expansion properties suchthat during thermal cycling and extended use, interfaces between layersretain interfacial strength. Solar cells formed in accordance with thepresent invention can have conversion efficiencies from about 18% toabout 25%, although further improved performances may be obtained byjudicious optimization and choice of materials based on the teachingsherein. For example, graded and/or multi-junction embodiments canpotentially realize efficiencies of up to about 10%, over conventionalsilicon solar cell efficiencies which are currently about 15-18%.

Current silicon semiconductor based solar cells (single crystal andpolycrystalline) typically include a deep buried grid of silver on thelight absorption side and a full faced aluminum layer intercepted bydeep buried grid of silver on the back side. As the silicon layerbecomes thinner, the processing temperature (e.g. 800° C.) for sinteringsilver powder to form a continuous mass and to diffuse silver across theanti-reflection layer on the top, and aluminum layer on back can causewarping. This is a result of the thermal expansion coefficient ofsilicon being much lower than that of silver or aluminum However the CTEof diamond-like carbon can be matched to that of silicon when theconductive diamond-like carbon replaces aluminum. Because aluminumgenerally covers the entire back side of the silicon layer, theconductive diamond-like carbon does not distort the thin silicon layer.Specifically, the silicon or semiconductor layer is not processed at aconventional high temperature, e.g. around 800° C. Further, diamond-likecarbon can form an excellent ohmic contact with silicon such thatadditional coating of silver does not require a high temperaturesintering of silver whereas only low temperature sintering is needed,e.g. less than about 300° C.

Further, solar cells formed in accordance with the present invention canalso include dye-sensitization such as by using dye-sensitized metaloxides and/or a layer of spectral sensitizing dye on a metal oxidesemiconductor layer in accordance with known dye sensitized solar celltechnologies. In yet another aspect of the present invention, thediamond-like carbon electronic device can be configured as a fuel cell.

Further, the entire diamond-like carbon electronic device can be a solidassembly having each layer in continuous intimate contact with adjacentlayers and/or members. The above-recited components can take a varietyof configurations and be made from a variety of materials. Each of thelayers can be formed using any number of known techniques such as, butnot limited to, vapor deposition, thin film deposition, preformedsolids, powdered layers, screen printing, or the like. In one aspect,each layer is formed using vapor deposition techniques such as PVD, CVD,or any other known thin-film deposition process. In one aspect, the PVDprocess is sputtering or cathodic arc.

Those of ordinary skill in the art will readily recognize othercomponents that can, or should, be added to the assembly of FIG. 2 inorder to achieve a specific purpose, or make a particular device. By wayof example, without limitation, a connecting line can be placed betweenthe anode and the cathode to form a complete circuit and allowelectricity to pass that can be used to power one or more devices (notshown), or perform other work.

In an optional step, the diamond-like carbon electronic devices can beheat treated in a vacuum furnace. Heat treatment can improve the thermaland electrical properties across the boundaries between differentmaterials. The diamond-like carbon electronic device can be subjected toa heat treatment to consolidate interfacial boundaries and reducematerial defects. Typical heat treatment temperatures can range fromabout 200° C. to about 800° C. and more preferably from about 350° C. toabout 500° C. depending on the specific materials chosen.

The following are examples illustrate various methods of makingelectronic devices in accordance with the present invention. However, itis to be understood that the following are only exemplary orillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative compositions, methods,and systems can be devised by those skilled in the art without departingfrom the spirit and scope of the present invention. The appended claimsare intended to cover such modifications and arrangements. Thus, whilethe present invention has been described above with particularity, thefollowing Examples provide further detail in connection with severalspecific embodiments of the invention.

EXAMPLE 1

FIG. 3 illustrates a graded solar cell in accordance with the presentinvention. A chemical vapor deposition silicon film 40 of about 10 μmhaving micron grains serves as the photoelectric substrate. The siliconfilm is coated successively with silicon-carbon layers via chemicalvapor deposition. Layers 42, 44, 46, 48 and 50 have a Si:C ratio of10:90, 20:80, 30:70, 40:60 and 50:50 (SiC), respectively. Each layer isdoped such that the graded portion represents a single p-n junction. Aconductive diamond-like carbon film 52 having a thickness of about 10 μmis deposited on the SiC layer 50. The diamond-like carbon is depositedat partial vacuum of acetylene gas and 13.6 MHz to form the conductivediamond-like carbon which is also light transmissive. The resultingsolar cell has layers having bandgaps from 1.1 eV for silicon to 3.3 eVfor silicon carbide which covers most visible light, e.g. red about 1 eVwhile blue is about 3 eV.

Of course, it is to be understood that the above-described arrangementsare only illustrative of the application of the principles of thepresent invention. Numerous modifications and alternative arrangementsmay be devised by those skilled in the art without departing from thespirit and scope of the present invention and the appended claims areintended to cover such modifications and arrangements. Thus, while thepresent invention has been described above with particularity and detailin connection with what is presently deemed to be the most practical andpreferred embodiments of the invention, it will be apparent to those ofordinary skill in the art that numerous modifications, including, butnot limited to, variations in size, materials, shape, form, function andmanner of operation, assembly and use may be made without departing fromthe principles and concepts set forth herein.

What is claimed is:
 1. A diamond-like carbon electronic device,comprising: a) a diamond-like carbon cathode including a conductivediamond-like carbon material having an sp^(a) bonded carbon content fromabout 30 atom % to about 90 atom %, a hydrogen content from 0 atom % toabout 30 atom %, and an sp² bonded carbon content from about 10 atom %to about 70 atom %; b) a charge carrier separation layer adjacent thediamond-like carbon cathode; and c) an anode adjacent the charge carrierseparation layer opposite the diamond-like carbon cathode.
 2. The deviceof claim 1, wherein the conductive diamond-like carbon material furtherincludes a dopant selected from the group consisting of B, N, Si, P, Li,or combinations thereof
 3. The device of claim 1, wherein the sp² bondedcarbon content is sufficient to provide the conductive diamond-likecarbon material with a visible light transmissivity of greater thanabout 0.70.
 4. The device of claim 1, wherein the sp² bonded carboncontent is from about 35 atom % to about 60 atom %.
 5. The device ofclaim 1, wherein the hydrogen content is from about 15 atom % to about25 atom %.
 6. The device of claim 1, wherein at least one of thediamond-like carbon cathode and anode further include a conductiveadditive selected from the group consisting of conductive metalparticles, carbon nanotubes, graphite, conductive nanoparticles, andcombinations thereof
 7. The device of claim 1, wherein the chargecarrier separation layer is a semiconductor layer.
 8. The device ofclaim 1, wherein the charge carrier separation layer comprises at leasta portion of a p-n or p-i-n junction and the diamond-like carbonelectronic device is a solar cell.
 9. The device of claim 8, wherein thesemiconductor layer comprises a member selected from the groupconsisting of silicon, gallium arsenide, gallium indium phosphide,gallium indium nitride, copper indium diselenide, cadmium telluride, andcomposites or combinations thereof
 10. The device of claim 9, whereinthe semiconductor layer comprises silicon.
 11. The device of claim 8,wherein the charge carrier separation layer further comprises multiplep-n or p-i-n junctions to form a multi-junction solar cell.
 12. Thedevice of claim 10, wherein the charge carrier separation layer furtherincludes an n-doped region and a p-doped region.
 13. The device of claim10, wherein the charge carrier separation layer includes a p-dopedregion and the diamond-like carbon cathode includes an n-doped region.14. The device of claim 8, wherein the charge carrier separation layeris substantially planar.
 15. The device of claim 8, wherein thediamond-like carbon cathode is a single layer and is configured as apassivation layer and an anti-reflection layer.
 16. The device of claim12, wherein the diamond-like carbon electronic device is a fuel cell.17. The device of claim 1, wherein the charge separation carrier layercomprises a crystalline material.
 18. The device of claim 1, wherein thecharge carrier separation layer is compositionally graded.
 19. Thedevice of claim 1, wherein the charge carrier separation layer has athickness from about 10 μm to about 300 μm.
 20. A method of forming adiamond-like carbon electronic device, comprising: a) preparing a chargecarrier separation layer; b) forming an anode, said anode being coupledadjacent the charge carrier separation layer; c) forming a diamond-likecarbon cathode coupled to the charge carrier separation layer oppositethe anode, said diamond-like carbon cathode including a diamond-likecarbon material having a conductivity from about 0 μΩ-cm to about 80μΩ-cm at 20° C. and a visible light transmissivity from about 0.5 toabout 1.0, said conductivity and visible light transmissivity being afunction of sp² and sp^(a) bonded carbon content, hydrogen content, andoptional conductive additives.